An image sensor is a semiconductor device for converting optical images into electrical signals and can be chiefly categorized into complementary metal oxide semiconductor (CMOS) image sensors and charge coupled devices (CCD).
CCDs may include individual metal oxide semiconductor (MOS) capacitors that are very closely located and charge carriers that may be stored and transferred in the capacitors. CMOS image sensors may detect outputs in a switching manner. In the CMOS image sensor, MOS transistors corresponding to the number of pixels are manufactured using CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits, and the outputs sequentially detected using the MOS transistors.
The CCD has a complicated driving method and exhibits high power consumption. Also, the CCD requires numerous masks (e.g., about 30-40) process steps, and thus, the process may be complicated. Even still, a signal processing circuit cannot be realized inside a CCD chip, which makes it difficult to form the CCD in a single chip.
In order to overcome these limitations, a CMOS image sensor using sub-micron CMOS technology is under active development. In the CMOS image sensor, a photodiode and a MOS transistor are formed within a unit pixel to sequentially detect signals in a switching manner to realize an image. CMOS image sensors require low power consumption and may require about twenty masks during processing, and thus, is very simple in fabrication in comparison to CCD processing. Also, since a variety of processing circuits can be incorporated into a single chip, the CMOS image sensor is in the lame light as a next generation image sensor and is used for a variety of application fields such as digital still cameras (DSC), personal computer (PC) cameras, and mobile cameras.